In recent years, a decrease in size and weight, an increase in power output and an increase in efficiency have been required in a permanent magnet motor used in a hybrid car, a hard disk drive, or the like. To realize such a decrease in size and weight, an increase in power output and an increase in efficiency in the permanent magnet motor mentioned above, film-thinning and a further improvement in magnetic performance are required of a permanent magnet to be buried in the permanent magnet motor. Meanwhile, as permanent magnet, there have been known ferrite magnets, Sm—Co-based magnets, Nd—Fe—B-based magnets, Sm2Fe17Nx-based magnets or the like. As permanent magnet for permanent magnet motor, there are typically used Nd—Fe—B-based magnets due to remarkably high residual magnetic flux density.
As method for manufacturing a permanent magnet, a powder sintering process is generally used. In this powder sintering process, raw material is coarsely milled first and furthermore, is finely milled into magnet powder by a jet mill (dry-milling method) or a wet bead mill (wet-milling method). Thereafter, the magnet powder is put in a mold and pressed to form in a desired shape with magnetic field applied from outside. Then, the magnet powder formed and solidified in the desired shape is sintered at a predetermined temperature (for instance, at a temperature between 800 and 1150 degrees Celsius for the case of Nd—Fe—B-based magnet) for completion.
On the other hand, as to Nd-based magnets such as Nd—Fe—B magnets, poor heat resistance is pointed to as defect. Therefore, in case a Nd-based magnet is employed in a permanent magnet motor, continuous driving of the motor brings the magnet into gradual decline of coercive force and residual magnetic flux density. Then, in case of employing a Nd-based magnet in a permanent magnet motor, in order to improve heat resistance of the Nd-based magnet, Dy (dysprosium) or Tb (terbium) having high magnetic anisotropy is added to further improve coercive force.